High-toughness cobalt-based alloy and engine valve coated with same

ABSTRACT

A surface hardening material being excellent in impact resistance and having abrasion resistance is provided. Provided are: a high-toughness cobalt-based alloy containing 25.0 to 40.0 mass % of Cr, 0.5 to 12.0 mass % of a sum of W and/or Mo, 0.8 to 5.5 mass % of Si, and 0.5 to 2.5 mass % of B, 8.0 mass % or less of each of Fe, Ni, Mn, and Cu, and 0.3 mass % or less of C, the sum amount of Fe, Ni, Mn, and C being 10.0 mass % or less, and the remainder comprising 48.0 to 68.0 mass % of Co and unavoidable impurities; and an engine valve coated with the same.

This application is a national stage of International Application No.:PCT/JP2011/061747, which was filed on May 23, 2011, and which claimspriority to JP2010-250597, which was filed on Nov. 9, 2010, and whichare both herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a surface hardening material used invarious construction processes such as PTA powder overlay welding, TIGoverlay welding, and thermal spraying, and more particularly to ahigh-toughness cobalt-based alloy used in a member that demands animpact resistance and to an engine valve coateded with the same.

BACKGROUND ART

Conventionally, in an engine valve, a Co—Cr—W—C type materialrepresented by Stellite® (including patent documents 1, 2) and aCo—Mo—Si type material represented by Tribaloy® (including patentdocuments 3 to 7) are used, and particularly Stellite is used in amember that demands an impact resistance.

The Co—Cr—W—C type material has an insufficient abrasion resistancethough having a high toughness as a surface hardening material, so thatthe material cannot withstand use for a long period of time. Also, theCo—Mo—Si type material has an insufficient impact resistance thoughhaving a high abrasion resistance as a surface hardening material, sothat problems such as cracks and exfoliation accompanied thereby areraised.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2001-123238

Patent Document 2: JP-A-2008-522039

Patent Document 3: Japanese Patent No. 4463763

Patent Document 4: JP-A-05-131289

Patent Document 5: JP-A-09-296241

Patent Document 6: JP-A-07-278780

Patent Document 7: JP-A-04-107235

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Surface hardening aiming at abrasion resistance or impact resistance iscurrently used widely in various fields of industry. However, due to animprovement in recent years in the consciousness to the environment,there is an increasing severity in the environment for use of industrialapparatus. In accompaniment therewith, a surface hardening material isrequested to have a higher performance. In particular, in a fillingmaterial used in an engine valve, development of a surface hardeningmaterial being excellent in impact resistance and having abrasionresistance is demanded.

Means for Solving the Problems

In the present invention, in a study of an alloy composition fordeveloping a surface hardening material excellent in impact resistance,the following target values have been set, and the conditions have beenset to satisfy all of these.

(Target Value)

(1) Charpy impact value→10 J/cm² or more

(2) Hardness→48 HRC or more

(3) Overlaying workability→the fluidity of a molten pool formed at thetime of overlay welding is good

(4) Thermal impact value→cracks or exfoliation is not generated until700° C.

In other words, a high-toughness cobalt-based alloy of the presentinvention that satisfies the above targets (1) to (4) contains 25.0 to40.0 mass % of Cr, 0.5 to 12.0 mass % of a sum of W and/or Mo, 0.8 to5.5 mass % of Si, and 0.5 to 2.5 mass % of B, wherein the remaindercomprises 48.0 to 68.0 mass % of Co and unavoidable impurities. Here,the unavoidable impurities are impurities that are mingled unavoidablyin the steps of producing each material and the like, though notintentionally added. Such impurities include, for example, Mg, S, O, N,V, Zr, Sn, and the like, and the total sum of these is typically 0.3mass % or less, which is of a degree that does not affect the functionof the present invention.

Also, a high-toughness cobalt-based alloy of the present inventioncontains 25.0 to 40.0 mass % of Cr, 0.5 to 12.0 mass % of a sum of Wand/or Mo, 0.8 to 5.5 mass % of Si, and 0.5 to 2.5 mass % of B, andfurther contains 8.0 mass % or less of each of Fe, Ni, Mn, and 0.3 mass% or less of C, wherein the sum amount of Fe, Ni, Mn, Cu, and C is 10.0mass % or less, and the remainder comprises 48.0 to 58.0 mass % of Coand unavoidable impurities.

Also, an engine valve of the present invention is filled or coated witha high-toughness cobalt-based alloy having the aforesaid composition. Amore preferable engine valve is such that at least a face surfaceabutting against a sheet is filled or coated with the aforesaidhigh-toughness cobalt-based alloy.

Next, the reason why the range of each component of the high-toughnesscobalt-based alloy pertaining to the present invention alloy is limitedwill be described.

Cr imparts abrasion resistance, impact resistance, corrosion resistance,and oxidation resistance by being dissolved into a Co solid solutionwhich serves as a substrate (matrix). Also, part of Cr forms anintermetallic compound with B so as to be dispersed in the matrix,thereby further contributing to improvement in the abrasion resistance.However, when the content is less than 25.0 mass, sufficient abrasionresistance, corrosion resistance, and oxidation resistance cannot beobtained. When the content exceeds 40.0 mass %, the intermetalliccompound is formed in a large amount in the metal tissue, therebygreatly lowering the toughness. Therefore, the range has been set to be25.0 to 40.0 mass %.

Mo and W impart abrasion resistance and impact resistance by beingdissolved into a Co solid solution, and part thereof forms anintermetallic compound with B, thereby further contributing toimprovement in the abrasion resistance. However, when the sum of Mo andW is less than 0.5 mass %, the solid solution reinforcement isinsufficient. When the content exceeds 12.0 mass %, the intermetalliccompound is formed in a large amount, thereby greatly lowering thetoughness (impact resistance). For this reason, the sum of Mo and/or Whas been set to be within a range of 0.5 to 12.0 mass %.

Si cleans the molten metal and improves the fluidity at the time offilling by being dissolved into a Co solid solution and compositelyadded together with B, thereby improving the construction workabilitysuch as PTA, TIG overlaying, or thermal spraying. However, when thecontent is less than 0.8 mass %, it does not contribute to solidsolution reinforcement, and moreover, the function of cleaning themolten metal will be insufficient. Also, when the content exceeds 5.5mass %, an intermetallic compound is formed, thereby greatly loweringthe toughness (impact resistance). For this reason, the content of Sihas been set to be within a range of 0.8 to 5.5 mass %.

B imparts abrasion resistance by forming an intermetallic compound withCr or Mo, and cleans the molten metal to improve the fluidity at thetime of filling, thereby improving the construction workability such asPTA, TIG overlaying, or thermal spraying. However, when the content isless than 0.5 mass %, a sufficient amount of intermetallic compound isnot formed, so that the abrasion property is lowered, and moreover, thefunction of cleaning the molten metal is insufficient. On the otherhand, when the content exceeds 2.5 mass %, the intermetallic compound islarge in amount, thereby greatly lowering the toughness (impactresistance). For this reason, the content of B has been set to be withina range of 0.5 to 2.5 mass %.

The cobalt-based alloy of the present invention can further contain 8.0mass % or less of each of Fe, Ni, Mn, and Cu, and 0.3 mass % or less ofC as added elements that do not give adverse effects on the physicalproperties. However, in order not to deteriorate the impact resistanceand abrasion resistance, the upper limit value of the sum amount of Fe,Ni, Mn, Cu, and C has been set to be 10.0 mass % or less.

Effect of the Invention

The cobalt-based alloy of the present invention has characteristics of aCharpy impact value of 10 J/cm² or more and a hardness of 48 HRC ormore, has a good PTA overlaying workability, and does not generatecracks or exfoliation until 700° C. in a thermal impact test of anoverlay test piece, so that the alloy can be applied to a wide range ofuse as a surface hardening material having excellent impact resistanceand abrasion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view in one example of an engine valve coated witha high-toughness cobalt-based alloy of the present invention.

MODE FOR CARRYING OUT THE INVENTION

The cobalt-based alloy of the present invention can be obtained byheating and melting an ingot, in which Co as a base, Cr, Mo, W, B, andSi as added components, and Fe, Ni, Mn, Cu, and C added in accordancewith the needs are each adjusted and blended to have a predeterminedmass %, in a crucible within a melting furnace so as to form an alloy ina liquid form, and thereafter transforming the molten alloy into apowder by the atomization method or the melt-crushing method or castingin a predetermined mold to form a rod shape or a plate shape.

In particular, the cobalt-based alloy powder produced by the atomizationmethod can be applied to surface reforming construction such as PTAoverlaying or thermal spraying by being adjusted to have a particle sizesuitable for the intended construction method. The overlay-welded rodproduced by the continuous casting method can be applied to surfacereforming construction such as gas overlaying. Also, by constructingthis powder or rod material on a face surface of an engine valve havingvarious material qualities, an abrasion resistance can be imparted tothe engine valve.

EXAMPLES

The alloys of the Examples of the present invention prepared and blendedin the above-described manner and the alloys of the Comparative Exampleswere melted and subjected to measurement of the Charpy impact value andthe Rockwell hardness by the methods described below. Also, anoverlaying test and a thermal impact test were carried out by themethods described below.

(1) Charpy Impact Test;

By using an electric furnace, 100 g of an ingot having a blendingcomposition of each alloy was heated to about 1600° C. and melted in astream of argon, and was cast into a shell mold to be mechanicallyprocessed into a test piece (without a notch) described in JIS Z2242:2005. Then, by using a Charpy impact tester, an impact test wascarried out in accordance with JIS Z 2242:2005, so as to measure theimpact value.

(2) Hardness Measurement;

A cast piece melted by the same method as described in the above (1) wasmechanically processed into a shape of 10×10×20 mm to expose a parallelsurface, and the top part of the parallel surface was subjected to wetpolishing with a water-resistant abrasive paper of No. 240, so as tocarry out a Rockwell hardness test in accordance with JIS Z 2245:2005.Here, the measurement was carried out in C scale.

(3) PTA Overlaying Test;

By using a high-frequency melting furnace, 100 kg of an ingot having ablending composition of each alloy was heated to about 1600° C. andmelted in a stream of argon, and was pulverized by the gas atomizationmethod, so as to adjust the particle size to 80 to 350 mesh by avibration sieve. This alloy powder was subjected to plasma powderoverlay welding on a face surface (the part shown by the reference sign2 in FIG. 1) of an engine valve (umbrella diameter of about 70 mm, axialdiameter of about 14 mm, and a total length of about 300 mm) fabricatedby using a heat-resistant steel (SUH3) defined in JIS G 4311:1991, andthe fluidity state of the molten pool formed at the time of overlaywelding was examined. Here, the evaluation standard for the fluiditystate is represented by any one of the following, and an alloy providinga good fluidity state is expressed as “Extremely good” or “Good”.

(Evaluation Standard)

Extremely good: equivalent to or more than Comparative Examples P, Q(cobalt self-fluxing alloy)

Good: equivalent to or more than Comparative Example A (Stellite 6)

Slightly poor: poorer than Comparative Example A but better thanComparative Example F (Tribaloy 400)

Poor: equivalent to or less than Comparative Example F

(4) Thermal Impact Test;

The engine valve fabricated by the same method as described in the above(3) was thermally treated for one hour in an electric furnace set at400° C., and thereafter out into water of about 20° C. for quickquenching. Subsequently, the thermal treatment temperature is furtherraised by 50° C., so as to perform a thermal treatment for one hour andquenching with water. This is repeated until the thermal treatmenttemperature becomes 700° C., and whether cracks or exfoliation wasgenerated or not after the thermal treatment at 700° C. was examined.

Table 1 shows the Examples of the present invention, and Table 2 andTable 3 show the Comparative Examples.

TABLE 1 Thermal impact test Impact (700° C.→quenching with water)Composition (mass %) value Hardness Overlay workability Presence orabsence of No. Co Cr B Si W Mo Others J/cm² HRC State of molten poolcracks or exfoliation Alloys 1 67.5 25.5 1.0 5.2 0.8 — — 11 51.5 Goodnone of the 2 52.5 39.0 1.5 1.0 3.0 3.0 — 17 50.1 Extremely good nonepresent 3 65.2 28.0 2.3 1.5 — 3.0 — 10 52.4 Extremely good noneinvention 4 55.5 30.0 1.5 2.0 5.5 5.5 — 10 50.8 Extremely good none 559.3 33.0 0.7 4.0 1.5 1.5 — 21 49.3 Good none 6 56.7 35.0 1.5 1.8 5.0 —— 16 49.3 Extremely good none 7 56.9 35.0 1.5 1.6 — 5.0 — 11 49.0Extremely good none 8 58.8 30.0 1.2 4.0 6.0 — — 12 48.3 Good none 9 60.135.0 1.8 1.6 0 1.5 — 14 49.2 Extremely good none 10 61.9 27.0 1.7 2.45.0 2.0 — 12 51.3 Extremely good none 11 49.5 30.0 1.5 2.0 4.0 6.0 7Fe11 49.7 Extremely good none 12 54.5 30.0 1.5 2.0 5.0 — 7Mn 17 48.3Extremely good none 13 48.5 34.0 1.0 3.5 2.0 2.0 4.5Ni, 4.5Cu 12 50.6Good none 14 57.2 35.0 1.3 2.3 4.0 — 0.2C 14 49.3 Extremely good none 1548.3 38.0 1.5 2.0 5.0 — 5Mn, 0.2C 10 52.2 Extremely good none 16 57.430.0 2.1 2.5 5.0 — 3Fe 12 52.4 Extremely good none 17 53.7 28.0 1.4 1.9— 10.0 5Fe 10 51.2 Extremely good none 18 49.3 32.0 1.5 2.2 10.0 — 5Ni12 50.7 Extremely good none

TABLE 2 Thermal impact test (700° C.→quenching Impact wit water)Composition (mass %) value Hardness Overlay workability Presence orabsence of No. Co Cr B Si W Mo Others J/cm² HRC State of molten poolcracks or exfoliation Comparative a 68.3 24.0 1.2 3.5 3.0 — — 22 44.8Extremely good none Examples b 51.0 42.0 1.5 1.5 — 4.0 — 5 54.7Extremely good present c 46.5 37.0 0.5 6.0 5.0 5.0 — 27 41.6 Good none d57.8 30.0 2.7 2.5 7.0 — — 4 55.0 Extremely good present e 67.5 28.0 2.00.5 2.0 — — 13 46.9 Extremely good none f 63.6 32.0 1.4 3.0 — — — 1946.1 Extremely good none g 52.8 30.0 2.2 2.0 5.0 8.0 — 7 51.3 Extremelygood present h 55.2 26.0 1.5 2.3 — 5.0 10Ni 17 43.5 Extremely good nonei 47.6 34.0 1.8 1.6 3.0 — 6Fe, 6Ni 18 45.3 Extremely good none j 52.738.0 0.8 3.0 2.5 2.5 0.5C 7 48.0 Good present

TABLE 3 Thermal impact test Overlay (700° C.→quenching Impactworkability with water) Composition (mass %) value Hardness State ofPresence or absence No. Co Mo W Cr B C Si Others J/cm² HRC molten poolof cracks or exfoliation Comparative A 65.7 — 4.6 28.5 — 1.2 — — 14 43.3Good none Examples B 60.0 — 8.5 30.0 — 1.5 — — 6 48.4 Good present C60.49 8.5 — 29.0 — 2.0 — 0.01Mn 7 48.2 Slightly poor present D 64.5 4.5— 29.0 — 1.25 — 0.55Mn, 0.2Ti 15 42.1 Poor none E 58.2 12.0 — 28.0 — 0.3— 1.5Ni 14 39.5 Slightly poor none F 59.7 29.0 — 8.5 — — 2.8 — 3 55.2Poor present G 59.5 22.0 — 17.0 — 0.2 1.3 — 4 49.2 Slightly poor presentH 56.4 27.0 — 14.0 — — 2.6 — 3 52.5 Slightly poor present I 60.32 28.5 —8.5 — 0.08 2.6 — 2 51.6 Slightly poor present J 50.228 28.6 — 17.7 0.0020.07 3.4 — 2 54.1 Poor present K 55.2 32.5 — 8.0 — 0.42 3.88 — 3 58.4Slightly poor present L 59.23 28.7 — 9.3 0.02 0.19 2.56 — 4 51.9 Poorpresent M 38.68 40.1 — 19.1 — 0.02 2.1 — 1 x Poor present N 59.46 33.7 —5.3 — 0.04 1.5 — 3 56.2 Slightly poor present O 59.798 21.4 — 16.6 —0.0023 2.2 — 5 49.6 Slightly poor none P 40.6 6.0 — 18.5 3.4 — 3.5 28Ni3 50.0 Extremely present good Q 66.8 — 8.0 20.0 2.7 — 2.5 — 4 57.0Extremely none good ※┌x┘ shows that the measurement could not beachieved since the alloy was fragile.

The alloys (a) to (j) of the Comparative Examples shown in Table 2 arealloys having a composition outside of the scope of the claims of thepresent invention. In the alloy (a), Co exceeds the upper limit of theclaims, and Cr is below the lower limit. In the alloy (c), Co is belowthe lower limit of the claims, and Si exceeds the upper limit. In thiscase, the hardness is lowered. Also, in the alloy (e), Si is below thelower limit of the claims. The alloy (f) contains neither of W nor Mo.In the alloy (h), Ni exceeds the upper limit of the claims and, in thealloy (i), the sum amount of Fe and Ni exceeds the upper limit of theclaims. In this case also, the hardness does not satisfy the targetvalue. In the alloy (b), Cr exceeds the upper limit of the claims; inthe alloy (d), B exceeds the upper limit; in the alloy (g), the sum ofMo and W exceeds the upper limit; and in the alloy (j), C exceeds theupper limit. In this case, the impact value does not satisfy the targetvalue in any of these. In the Comparative Examples (b), (d), (g), and(j) in which the target of the Charpy impact value is not satisfied,generation of cracks was confirmed in the thermal impact test.

The alloys (A) to (Q) of the Comparative Examples shown in Table 3 areconventional Co—Cr—W—C type alloys (Stellite®, Co—Mo—Si type alloys(Tribaloy®, or alloys having a composition shown in other patentdocuments or self-fluxing alloy thermal spraying of JIS H 8303; however,none of these satisfies the target value of either the impact value orthe hardness. Further, in the Comparative Examples (C) to (O), theoverlay workability is poor. Also, in the Comparative Examples (B), (C),(F) to (N), and (P), generation of cracks was confirmed in the thermalimpact test.

On the other hand, as shown in Table 1, the Examples 1 to 18 which arethe alloys of the present invention satisfy the target values of all ofthe Charpy impact value, the Rockwell hardness, and the thermal impacttest, and have a suitable hardness and a high toughness as a surfacehardening material, and further are excellent in overlay workability.

INDUSTRIAL APPLICABILITY

As described above, the cobalt-based alloy provided by the presentinvention has excellent abrasion resistance and impact resistanceproperties as a surface hardening material, and also satisfies thefluidity property. Therefore, by forming such a material into a powdershape or a rod shape and performing surface hardening on various membersby welding or thermal spraying, these members can be expected to enjoy aconsiderable improvement in durability.

Also, the cobalt-based alloy of the present invention is not limited tousage for filling, so that, by compositely adding this into a sinteredcomponent formed by powder metallurgy to form hard particles, this canbe utilized also for forming a mechanical component having a goodabrasion resistance.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 engine valve    -   2 face surface (surface abutting against a sheet)

The invention claimed is:
 1. A high-toughness cobalt-based alloy forsurface hardening of an engine valve, consisting of: 25.0 to 40.0 mass %of Cr; 3.0 to 12.0 mass % of Mo; 0.8 to 5.5 mass % of Si; 0.5 to 2.5mass % of B; and a remainder being 48.0 to 68.0 mass % of Co andunavoidable impurities, wherein the high-toughness cobalt-based alloyhas a Charpy impact value of 10 J/cm² or more and a hardness of 48 HRCor more.
 2. An engine valve filled or coated with a high-toughnesscobalt-based alloy according to claim
 1. 3. The engine valve accordingto claim 2, wherein at least a face surface abutting against a sheet isfilled or coated with said high-toughness cobalt-based alloy.
 4. Ahigh-toughness cobalt-based alloy for surface hardening of an enginevalve, consisting of: 25.0 to 40.0 mass % of Cr; 0.8 to 5.5 mass % ofSi; 0.5 to 2.5 mass % of B; at least one selected from Fe and Ni; and aremainder being 48.0 to 68.0 mass % of Co and unavoidable impurities,wherein a sum amount of Fe and Ni is 10.0 mass % or less, a content ofFe is 8.0 mass % or less, and a content of Ni is 8.0 mass % or less, andwherein the high-toughness cobalt-based alloy has a Charpy impact valueof 10 J/cm² or more and a hardness of 48 HRC or more.
 5. An engine valvefilled or coated with a high-toughness cobalt-based alloy according toclaim
 4. 6. The engine valve according to claim 5, wherein at least aface surface abutting against a sheet is filled or coated with saidhigh-toughness cobalt-based alloy.